1. Field of the Invention
The present invention relates to a vehicle-body stiffening device that uses traveling wind to apply a preload to a stiffening member, which connects regions of a vehicle body to each other, so as to increase the rigidity of the vehicle body.
2. Description of the Related Art
It is commonly known that when a vehicle, such as an automobile, is cornering or slaloming at high speed, a vertical pressing force on a support portion that supports a suspension is transmitted to the vehicle body without being sufficiently absorbed by the suspension, causing bending deformation to occur in the vehicle body. In this case, if the rigidity of the vehicle body is low, a phase lag in the bending deformation (vehicle-body hysteresis) occurs significantly, giving the driver an impression of poor steering responsiveness (vehicle tracking properties) due to a lack of unity between steering maneuverability and vehicle behavior.
Although increasing the rigidity of the vehicle body can reduce the vehicle-body hysteresis, this can lead not only to a higher price of the product as well as lower fuel efficiency due to an increased vehicle weight, but also to reduced riding comfort due to an increased sense of ruggedness when the vehicle is running.
In light of this, a technique of locally increasing the rigidity of relevant regions of the vehicle body by stiffening these relevant regions using stiffening members is widely employed. For example, Japanese Unexamined Patent Application Publication No. 2008-290552 (hereinafter referred to as “Patent Document 1”) discloses a vehicle-body stiffening device in which an expandable-contractible stiffening member (connecting member) connects shafts provided at opposite ends of a suspension cross member in the vehicle-width direction and supporting bases of suspension arms so that the rigidity of the suspension cross member can be increased. In addition, the vehicle-body stiffening device includes biasing means configured to constantly bias the shafts toward each other by applying a preload (kg/mm) thereto using a spring pressure.
Japanese Unexamined Patent Application Publication No. 2006-182133 (hereinafter referred so as “Patent Document 2”) discloses a vehicle-body stiffening device that locally increases the rigidity of the vehicle body by using a stiffening member (strut, tower bar) to connect strut, support portions that support upper sections of strut, suspensions provided at left and right sides in the vehicle-width direction. In addition, in the vehicle-body stiffening device, biasing means configured to apply a bias force in the expanding-contracting direction by using a gas pressure is provided at an intermediate position of a main bar portion so as constantly apply a preload between the strut support portions. Moreover, Patent Document 2 also discloses a technique of attenuating the expansion and contraction of the stiffening member by using an oil damper provided in the biasing means.
With the vehicle-body stiffening device disclosed in each of Patent Documents 1 and 2, since the biasing means constantly applies a preload to the members spaced apart from each other, the vehicle-body hysteresis can be effectively reduced.
However, in the vehicle-body stiffening device equipped with the biasing means disclosed in each of Patent Documents 1 and 2 described above, the device needs to be installed between the aforementioned members in a state where the preload generated by the biasing means is suppressed or in a state where a jig that suppresses the preload is attached. The former involves a time-consuming installation process, whereas the latter requires a process for removing the jig after the installation. In either case, the process is complicated. Moreover, the preload generated by the biasing means tends to weaken with time, which is problematic in terms of durability.
Although the vehicle-body hysteresis maintains its initial state when the vehicle is at a stop and is thus substantially 0 (%), the vehicle-body hysteresis increases with increasing vehicle speed. However, because the preload is set at a fixed value by the biasing means in the techniques disclosed in Patent Documents 1 and 2 described above, if the steering responsiveness in a high-speed running mode is to be enhanced by, for example, setting the preload characteristics to a high-speed side, the preload is excessively applied, to the vehicle body during a low-speed to mid-speed running mode. This results in reduced riding comfort when the vehicle is running at low to mid speed.